This invention provides compositions and methods for the recovery of hydrocarbon fluids from a subterranean reservoir. More particularly, this invention concerns expandable cross-linked polymeric microparticle compositions that modify the permeability of subterranean formations and increase the mobilization and/or recovery rate of hydrocarbon fluids present in the formations.
In the first stage of hydrocarbon recovery the sources of energy present in the reservoir are allowed to move the oil, gas, condensate etc. to the producing wells(s) where they can flow or be pumped to the surface handling facility. A relatively small proportion of the hydrocarbon in place can usually be recovered by this means. The most widely used solution to the problem of maintaining the energy in the reservoir and ensuring that hydrocarbon is driven to the producing well(s) is to inject fluids down adjacent wells. This is commonly known as secondary recovery.
The fluids normally used are water (such as aquifer water, river water, sea water, or produced water), or gas (such as produced gas, carbon dioxide, flue gas and various others). If the fluid encourages movement of normally immobile residual oil or other hydrocarbon, the process is commonly termed tertiary recovery.
A very prevalent problem with secondary and tertiary recovery projects relates to the heterogeneity of the reservoir rock strata. The mobility of the injected fluid is commonly different from the hydrocarbon and when it is more mobile various mobility control processes have been used to make the sweep of the reservoir more uniform and the consequent hydrocarbon recovery more efficient. Such processes have limited value when high permeability zones, commonly called thief zones or streaks, exist within the reservoir rock. The injected fluid has a low resistance route from the injection to the production well. In such cases the injected fluid does not effectively sweep the hydrocarbon from adjacent, lower permeability zones. When the produced fluid is rexe2x80x94used this can lead to fluid cycling through the thief zone to little benefit and at great cost in terms of fuel and maintenance of the pumping system.
Numerous physical and chemical methods have been used to divert injected fluids out of thief zones in or near production and injection wells. When the treatment is applied to a producing well it is usually termed a water (or gas etc.) shutxe2x80x94off treatment. When it is applied to an injection well it is termed a profile control or conformance control treatment.
In cases where the thief zone(s) are isolated from the lower permeability adjacent zones and when the completion in the well forms a good seal with the barrier (such as a shale layer or xe2x80x9cstringerxe2x80x9d) causing the isolation, mechanical seals or xe2x80x9cplugsxe2x80x9d can be set in the well to block the entrance of the injected fluid. If the fluid enters or leaves the formation from the bottom of the well, cement can also be used to fill up the well bore to above the zone of ingress.
When the completion of the well allows the injected fluid to enter both the thief and the adjacent zones, such as when a casing is cemented against the producing zone and the cement job is poorly accomplished, a cement squeeze is often a suitable means of isolating the watered out zone.
Certain cases are not amenable to such methods by virtue of the facts that communication exists between layers of the reservoir rock outside the reach of cement. Typical examples of this are when fractures or rubble zones or washed out caverns exist behind the casing. In such instances chemical gels, capable of moving through pores in reservoir rock have been applied to seal off the swept out zones.
When such methods fail the only alternatives remaining are to produce the well with poor recovery rate, sidetrack the well away from the prematurely swept zone, or the abandon the well. Occasionally the producing well is converted to a fluid injector to increase the field injection rate above the net hydrocarbon extraction rate and increase the pressure in the reservoir. This can lead to improved overall recovery but it is worthy of note that the injected fluid will mostly enter the thief zone at the new injector and is likely to cause similar problems in nearby wells. All of these are expensive options.
Near wellbore conformance control methods always fail when the thief zone is in widespread contact with the adjacent, hydrocarbon containing, lower permeability zones. The reason for this is that the injected fluids can bypass the treatment and rexe2x80x94enter the thief zone having only contacted a very small proportion, or even none of the remaining hydrocarbon. It is commonly known amongst those skilled in the art, that such near wellbore treatments do not succeed in significantly improving recovery in reservoirs having crossflow of the injected fluids between zones.
A few processes have been developed with the aim of reducing the permeability in a substantial proportion of the thief zone and, or at a significant distance from the injection and production wells. One example of this is the Deep Diverting Gel process patented by Morgan et al (1). This has been used in the field and suffered from sensitivity to unavoidable variations in quality of the reagents which resulted in poor propagation. The gelant mixture is a two component formulation and it is believed that this contributed to poor propagation of the treatment into the formation.
The use of swellable cross linked superabsorbent polymer microparticles for modifying the permeability of subterranean formations is disclosed in U.S. Pat. Nos. 5,465,792 and 5,735,349. However, swelling of the superabsorbent microparticles described therein is induced by changes of the carrier fluid from hydrocarbon to aqueous or from water of high salinity to water of low salinity.
We have discovered novel polymeric microparticles in which the microparticle conformation is constrained by reversible (labile) internal crosslinks. The microparticle properties, such as particle size distribution and density, of the constrained microparticle are designed to allow efficient propagation through the pore structure of hydrocarbon reservoir matrix rock, such as sandstone. On heating to reservoir temperature and/or at a predetermined pH, the reversible (labile) internal crosslinks start to break allowing the particle to expand by absorbing the injection fluid (normally water).
The ability of the particle to expand from its original size (at the point of injection) depends only on the presence of conditions that induce the breaking of the labile crosslinker. It does not depend on the nature of the carrier fluid or the salinity of the reservoir water. The particles of this invention can propagate through the porous structure of the reservoir without using a designated fluid or fluid with salinity higher than the reservoir fluid.
The expanded particle is engineered to have a particle size distribution and physical characteristics, for example, particle rheology, which allow it to impede the flow of injected fluid in the pore structure. In doing so it is capable of diverting chase fluid into less thoroughly swept zones of the reservoir.
The rheology and expanded particle size of the particle can be designed to suit the reservoir target, for example by suitable selection of the backbone monomers or comonomer ratio of the polymer, or the degree of reversible (labile) and irreversible crosslinking introduced during manufacture.
Highly cross linked expandable polymeric microparticles having a labile cross linking agent content of from about 9,000 to about 200,000 ppm are disclosed in commonly assigned U.S. Pat. No. 6,454,003. However, for oil fields with a short fluid transit time between injection wells and the connecting producing wells, it would be necessary to inject microparticles that will swell in a shorter amount of time than those of U.S. Pat. No. 6,454,003. Also, for reservoirs of relatively low temperature, particles that will swell in a timely manner at that temperature are also required. We have discovered that polymeric microparticles made with lower levels of labile crosslinker are effective for these purposes.
Accordingly, this invention is a method of modifying the permeability to water of a subterranean formation comprising injecting into the subterranean formation a composition comprising highly cross linked expandable polymeric microparticles having an unexpanded volume average particle size diameter of from about 0.05 to about 10 microns and a cross linking agent content of about 1,000 to less than about 9,000 ppm of labile cross linkers and from 0 to about 300 ppm of non-labile cross linkers, wherein the microparticles have a smaller diameter than the pores of the subterranean formation and wherein the labile cross linkers break under the conditions of temperature and pH in the subterranean formation to form expanded microparticles.
Definitions of Terms
xe2x80x9cAmphoteric polymeric microparticlexe2x80x9d means a cross-linked polymeric microparticle containing both cationic substituents and anionic substitutents, although not necessarily in the same stoichiometric proportions. Representative amphoteric polymeric microparticles include terpolymers of nonionic monomers, anionic monomers and cationic monomers as defined herein. Preferred amphoteric polymeric microparticles have a higher than 1:1 anionic monomer/cationic monomer mole ratio.
xe2x80x9cAmpholytic ion pair monomer: means the acid-base salt of basic, nitrogen containing monomers such as dimethylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate (DMAEM), 2-methacryloyloxyethyldiethylamine, and the like and acidic monomers such as acrylic acid and sulfonic acids such as 2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, and the like.
xe2x80x9cAnionic monomerxe2x80x9d means a monomer as defined herein which possesses an acidic functional group and the base addition salts thereof. Representative anionic monomers include acrylic acid, methacrylic acid, maleic acid, itaconic acid, 2-propenoic acid, 2-methyl-2-propenoic acid, 2-acrylamido-2-methyl propane sulfonic acid, sulfopropyl acrylic acid and other water-soluble forms of these or other polymerizable carboxylic or sulphonic acids, sulphomethylated acrylamide, allyl sulphonic acid, vinyl sulphonic acid, the quaternary salts of acrylic acid and methacrylic acid such as ammonium acrylate and ammonium methacrylate, and the like. Preferred anionic monomers include 2-acrylamido-2-methyl propanesulfonic acid sodium salt, vinyl sulfonic acid sodium salt and styrene sulfonic acid sodium salt. 2-Acrylamido-2-methyl propanesulfonic acid sodium salt is more preferred.
xe2x80x9cAnionic polymeric microparticlexe2x80x9d means a cross-linked polymeric microparticle containing a net negative charge. Representative anionic polymeric microparticles include copolymers of acrylamide and 2-acrylamido-2-methyl propane sulfonic acid, copolymers of acrylamide and sodium acrylate, terpolymers of acrylamide, 2-acrylamido-2-methyl propane sulfonic acid and sodium acrylate and homopolymers of 2-acrylamido-2-methyl propane sulfonic acid. Preferred anionic polymeric microparticles are prepared from about 95 to about 10 mole percent of nonionic monomers and from about 5 to about 90 mole percent anionic monomers. More preferred anionic polymeric microparticles are prepared from about 95 to about 10 mole percent acrylamide and from about 5 to about 90 mole percent 2-acrylamido-2-methyl propane sulfonic acid.
Betaine-containing polymeric microparticlexe2x80x9d means a cross-linked polymeric microparticle prepared by polymerizing a betaine monomer and one or more nonionic monomers.
xe2x80x9cBetaine monomerxe2x80x9d means a monomer containing cationically and anionically charged functionality in equal proportions, such that the monomer is net neutral overall. Representative betaine monomers include N,N-dimethyl-N-acryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine, N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine, N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine, N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine, N,N-dimethyl-N-acryloxyethyl-N-(3-sulfopropyl)-ammonium betaine, N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine, N-3-sulfopropylvinylpyridine ammonium betaine, 2-(methylthio)ethyl methacryloyl-S-(sulfopropyl)-sulfonium betaine, 1-(3-sulfopropyl)-2-vinylpyridinium betaine, N-(4-sulfobutyl)-N-methyldiallylamine ammonium betaine (MDABS), N,N-diallyl-N-methyl-N-(2-sulfoethyl)ammonium betaine, and the like. A preferred betaine monomer is N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine.
xe2x80x9cCationic Monomerxe2x80x9d means a monomer unit as defined herein which possesses a net positive charge. Representative cationic monomers include the quaternary or acid salts of dialkylaminoalkyl acrylates and methacrylates such as dimethylaminoethylacrylate methyl chloride quaternary salt (DMAEA.MCQ), dimethylaminoethylmethacrylate methyl chloride quaternary salt (DMAEM.MCQ), dimethylaminoethylacrylate hydrochloric acid salt, dimethylaminoethylacrylate sulfuric acid salt, dimethylaminoethyl acrylate benzyl chloride quaternary salt (DMAEA.BCQ) and dimethylaminoethylacrylate methyl sulfate quaternary salt; the quaternary or acid salts of dialkylaminoalkylacrylamides and methacrylamides such as dimethylaminopropyl acrylamide hydrochloric acid salt, dimethylaminopropyl acrylamide sulfuric acid salt, dimethylaminopropyl methacrylamide hydrochloric acid salt and dimethylaminopropyl methacrylamide sulfuric acid salt, methacrylamidopropyl trimethyl ammonium chloride and acrylamidopropyl trimethyl ammonium chloride; and N,N-diallyldialkyl ammonium halides such as diallyldimethyl ammonium chloride (DADMAC). Preferred cationic monomers include dimethylaminoethylacrylate methyl chloride quaternary salt, dimethylaminoethylmethacrylate methyl chloride quaternary salt and diallyldimethyl ammonium chloride. Diallyldimethyl ammonium chloride is more preferred.
xe2x80x9cCross linking monomerxe2x80x9d means an ethylenically unsaturated monomer containing at least two sites of ethylenic unsaturation which is added to constrain the microparticle conformation of the polymeric microparticles of this invention. The level of cross linking used in these polymer microparticles is high, compared to conventional super-absorbent polymers, to maintain a rigid non-expandable microparticle configuration. Cross linking monomers according to this invention include both labile cross linking monomers and non-labile cross linking monomers.
xe2x80x9cEmulsionxe2x80x9d, xe2x80x9cmicroemulsionxe2x80x9d and xe2x80x9cinverse emulsionxe2x80x9d mean a water-in-oil polymer emulsion comprising a polymeric microparticle according to this invention in the aqueous phase, a hydrocarbon oil for the oil phase and one or more water-in-oil emulsifying agents. Emulsion polymers are hydrocarbon continuous with the water-soluble polymers dispersed within the hydrocarbon matrix. The emulsion polymers are optionally xe2x80x9cinvertedxe2x80x9d or converted into water-continuous form using shear, dilution, and, generally an inverting surfactant. See U.S. Pat. No. 3,734,873, incorporated herein by reference.
xe2x80x9cFluid mobilityxe2x80x9d means a ratio that defines how readily a fluid moves through a porous medium. This ratio is known as the mobility and is expressed as the ratio of the permeability of the porous medium to the viscosity for a given fluid.       1.    ⁢          xe2x80x83        ⁢    λ    =                    k        x                    η        x              ⁢          xe2x80x83        ⁢    for    ⁢          xe2x80x83        ⁢    a    ⁢          xe2x80x83        ⁢    single    ⁢          xe2x80x83        ⁢    fluid    ⁢          xe2x80x83        ⁢    x    ⁢          xe2x80x83        ⁢    flowing    ⁢          xe2x80x83        ⁢    in    ⁢          xe2x80x83        ⁢    a    ⁢          xe2x80x83        ⁢    porous    ⁢          xe2x80x83        ⁢          medium      .      
When more than one fluid is flowing the end point relative permeabilities must be substituted for the absolute permeability used in equation 1.             2.      ⁢              xe2x80x83            ⁢              λ        x              =                            k          rx                          η          x                    ⁢              xe2x80x83            ⁢      for      ⁢              xe2x80x83            ⁢      a      ⁢              xe2x80x83            ⁢      fluid      ⁢              xe2x80x83            ⁢      x      ⁢              xe2x80x83            ⁢      flowing      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      a      ⁢              xe2x80x83            ⁢      porous      ⁢              xe2x80x83            ⁢      medium      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      presence      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      one      ⁢              xe2x80x83            ⁢      or      ⁢                        xe2x80x83                ⁢                  xe2x80x83                    ⁢      more      ⁢              xe2x80x83            ⁢      other      ⁢              xe2x80x83            ⁢              fluids        .              ⁢      xe2x80x83  
When two or more fluids are flowing the fluid mobilities may be used to define a Mobility ratio       3.    ⁢          xe2x80x83        ⁢    M    =                    λ        x                    λ        y              =                            η          y                ⁢                  k          rx                                      η          x                ⁢                  k          ry                    
The mobility ratio is of use in the study of fluid displacement, for example in water flooding of an oil reservoir where x is water and y is oil, because the efficiency of the displacement process can be related to it. As a general principle at a mobility ratio of 1 the fluid front moves almost in a xe2x80x9cPlug flowxe2x80x9d manner and the sweep of the reservoir is good. When the mobility of the water is ten times greater than the oil viscous instabilities, known as fingering, develop and the sweep of the reservoir is poor. When the mobility of the oil is ten times greater than the water the sweep of the reservoir is almost total.
xe2x80x9cIon-pair polymeric microparticlexe2x80x9d means a cross-linked polymeric microparticle prepared by polymerizing an ampholytic ion pair monomer and one more anionic or nonionic monomers.
xe2x80x9cLabile cross linking monomerxe2x80x9d means a cross linking monomer which can be degraded by certain conditions of heat and/or pH, after it has been incorporated into the polymer structure, to reduce the degree of crosslinking in the polymeric microparticle of this invention. The aforementioned conditions are such that they can cleave bonds in the xe2x80x9ccross linking monomerxe2x80x9d without substantially degrading the rest of the polymer backbone. Representative labile cross linking monomers include diacrylamides and methacrylamides of diamines such as the diacrylamide of piperazine, acrylate or methacrylate esters of di, tri, tetra hydroxy compounds including ethyleneglycol diacrylate, polyethyleneglycol diacrylate, trimethylopropane trimethacrylate, ethoxylated trimethylol triacrylate, ethoxylated pentaerythritol tetracrylate, and the like; divinyl or diallyl compounds separated by an azo such as the diallylamide of 2,2xe2x80x2-Azobis(isbutyric acid) and the vinyl or allyl esters of di or tri functional acids. Preferred labile cross linking monomers include water soluble diacrylates such as PEG 200 diacrylate and PEG 400 diacrylate and polyfunctional vinyl derivatives of a polyalcohol such as ethoxylated (9-20) trimethylol triacrylate.
The labile cross linker is present in an amount of about 1,000 to less than about 9,000 ppm, preferably about 2,000 to about 9,000 ppm and more preferably about 3,000 to about 9,000 ppm based on total moles of monomer. The amount of labile cross linker needed depends on reservoir conditions such as temperature, pH of the reservoir brine, and the transit time of the injected fluid. The preferred amount is the amount that will allow the microparticles to expand sufficiently to impede the transit of the fluid within the resident time of the injection fluid within an injector-producer pair.
xe2x80x9cMonomerxe2x80x9d means a polymerizable allylic, vinylic or acrylic compound. The monomer may be anionic, cationic, nonionic or zwitterionic. Vinyl monomers are preferred, acrylic monomers are more preferred.
xe2x80x9cNonionic monomerxe2x80x9d means a monomer as defined herein which is electrically neutral. Representative nonionic monomers include N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, dimethylaminopropyl acrylamide, dimethylaminopropyl methacrylamide, acryloyl morpholine, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, dimethylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate (DMAEM), maleic anhydride, N-vinyl pyrrolidone, vinyl acetate and N-vinyl formamide. Preferred nonionic monomers include acrylamide, N-methylacrylamide, N,N-dimethylacrylamide and methacrylamide. Acrylamide is more preferred.
xe2x80x9cNon-labile cross linking monomerxe2x80x9d means a cross linking monomer which is not degraded under the conditions of temperature and/or pH which would cause incorporated labile cross linking monomer to disintegrate. Non-labile cross linking monomer is added, in addition to the labile cross linking monomer, to control the expanded conformation of the polymeric microparticle. Representative non-labile cross linking monomers include methylene bisacrylamide, diallylamine, triallylamine, divinyl sulfone, diethyleneglycol diallyl ether, and the like. A preferred non-labile cross linking monomer is methylene bisacrylamide.
The non-labile cross linker is present in an amount of from 0 to about 300 ppm, preferably from about 0 to about 200 ppm and more preferably from about 0 to about 100 ppm based on total moles of monomer. In the absence of a non-labile cross linker, the polymer particle, upon complete scission of labile cross linker, is converted into a mixture of linear polymer strands. The particle dispersion is thereby changed into a polymer solution. This polymer solution, due to its viscosity, changes the mobility of the fluid in a porous medium. In the presence of a small amount of non-labile cross linker, the conversion from particles to linear molecules is incomplete. The particles become a loosely linked network but retain certain xe2x80x98structurexe2x80x99. Such xe2x80x98structuredxe2x80x99 particles can block the pore throats of porous media and create a blockage of flow.
Preferred Embodiments
In a preferred aspect, the polymeric microparticles of this invention are prepared using an inverse emulsion or microemulsion process to assure certain particle size range. The unexpanded volume average particle size diameter of the polymeric microparticle is preferably from about 0.1 to about 3 microns, more preferably from about 0.1 to about 1 microns.
Representative preparations of cross-linked polymeric microparticles using microemulsion process are described in U.S. Pat. Nos. 4,956,400, 4,968,435, 5,171,808, 5,465,792 and 5,735,439.
In an inverse emulsion or microemulsion process, an aqueous solution of monomers and cross linkers is added to a hydrocarbon liquid containing an appropriate surfactant or surfactant mixture to form an inverse monomer microemulsion consisting of small aqueous droplets dispersed in the continuous hydrocarbon liquid phase and subjecting the monomer microemulsion to free radical polymerization.
In addition to the monomers and cross linkers, the aqueous solution may also contain other conventional additives including chelating agents to remove polymerization inhibitors, pH adjusters, initiators and other conventional additives.
The hydrocarbon liquid phase comprises a hydrocarbon liquid or mixture of hydrocarbon liquids. Saturated hydrocarbons or mixtures thereof are preferred. Typically, the hydrocarbon liquid phase comprises benzene, toluene, fuel oil, kerosene, odorless mineral spirits and mixtures of any of the foregoing.
Surfactants useful in the microemulsion polymerization process described herein include sorbitan esters of fatty acids, ethoxylated sorbitan esters of fatty acids, and the like or mixtures thereof. Preferred emulsifying agents include ethoxylated sorbitol oleate and sorbitan sesquioleate. Additional details on these agents may be found in McCutcheon""s Detergents and Emulsifiers, North American Edition, 1980.
Polymerization of the emulsion may be carried out in any manner known to those skilled in the art. Initiation may be effected with a variety of thermal and redox free-radical initiators including azo compounds, such as azobisisobutyronitrile; peroxides, such as t-butyl peroxide; organic compounds, such as potassium persulfate and redox couples, such as sodium bisulfite/sodium bromate. Preparation of an aqueous product from the emulsion may be effected by inversion by adding it to water which may contain an inverting surfactant.
Alternatively, the polymeric microparticles cross linked with labile cross links are prepared by internally cross linking polymer particles which contain polymers with pendant carboxylic acid and hydroxyl groups. The cross linking is achieved through the ester formation between the carboxylic acid and hydroxyl groups. The esterification can be accomplished by azeotropic distillation (U.S. Pat. No. 4,599,379) or thin film evaporation technique (U.S. Pat. No. 5,589,525) for water removal. For example, a polymer microparticle prepared from inverse emulsion polymerization process using acrylic acid, 2-hydroxyethylacrylate, acrylamide and
2-acrylamido-2-methylpropanesulfonate sodium as monomer is converted into cross linked polymer particles by the dehydration processes described above.
The polymeric microparticles are optionally prepared in dry form by adding the emulsion to a solvent which precipitates the polymer such as isopropanol, isopropanol/acetone or methanol/acetone or other solvents or solvent mixtures that are miscible with both hydrocarbon and water and filtering off and drying the resulting solid.
An aqueous suspension of the polymeric microparticles is prepared by redispersing the dry polymer in water.
In a preferred aspect of this invention, the microparticle contains about 2,000 ppm to less than about 9,000 ppm of labile cross linkers.
In another preferred aspect, the microparticle contains about 3,000 ppm to less than about 9,000 ppm of labile cross linkers.
The composition then flows through the a zone or zones of relatively high permeability in the subterranean formation under increasing temperature conditions, until the composition reaches a location where the temperature or pH is sufficiently high to promote expansion of the microparticles.
Unlike conventional blocking agents such as polymer solutions and polymer gels that cannot penetrate far and deep into the formation, the composition of this invention, due to the size of the particles and low viscosity, can propagate far from the injection point until it encounters the high temperature zone.
Also, the polymeric microparticles of this invention, due to their highly crosslinked nature, do not expand in solutions of different salinity. Consequently, the viscosity of the dispersion is not affected by the salinity of the fluid encountered in the subterranean formation. Accordingly, no special carrier fluid is needed for treatment. Only after the particles encounter conditions sufficient to reduce the crosslinking density, is the fluid rheology changed to achieve the desired effect.
Among other factors, the reduction in crosslinking density is dependent on the rate of cleavage of the labile crosslinker. In particular, different labile crosslinkers, have different rates of bond cleavage at different temperatures. The temperature and mechanism depend on the nature of the cross-linking chemical bonds. For example, when the labile crosslinker is PEG diacrylate, hydrolysis of the ester linkage is the mechanism of de-crosslinking. Different alcohols have slightly different rates of hydrolysis. In general, methacrylate esters will hydrolyze at a slower rate than acrylate esters under similar conditions. With divinyl or diallyl compounds separated by an azo group such as the diallylamide of 2,2xe2x80x2-Azobis(isbutyric acid), the mechanism of de-crosslinking is elimination of a nitrogen molecule. As demonstrated by various azo initiators for free radical polymerization, different azo compounds indeed have different half-life temperatures for decomposition.
In addition to the rate of de-crosslinking, we believe that the rate of particle diameter expansion also depends on the total amount of remaining crosslinking. We have observed that the particle expands gradually initially as the amount of crosslinking decreases. After the total amount of crosslinking passes below a certain critical density, the viscosity increases explosively. Thus, by proper selection of the labile cross-linker, both temperature- and time-dependent expansion properties can be incorporated into the polymer particles.
The particle size of the polymer particles before expansion is selected based on the calculated pore size of the highest permeability thief zone. The crosslinker type and concentration, and hence the time delay before the injected particles begin to expand, is based on the temperature both near the injection well and deeper into the formation, the expected rate of movement of injected particles through the thief zone and the ease with which water can crossflow out of the thief zone into the adjacent, lower permeability, hydrocarbon containing zones. A polymer microparticle composition designed to incorporate the above considerations results in a better water block after particle expansion, and in a more optimum position in the formation.
In a preferred aspect of this embodiment, the composition is added to injection water as part of a secondary or tertiary process for the recovery of hydrocarbon from the subterranean formation.
In another preferred aspect of this embodiment, the injection water is added to the subterranean formation at a temperature lower than the temperature of the subterranean formation.
A composition comprising about 100 to 10,000 ppm, preferably from about 500 to about 1500 ppm and more preferably from about 500 to about 1000 ppm based on microparticle actives is suitable for treating subterranean formations.
In another preferred aspect of this embodiment, the subterranean formation is a sandstone or carbonate hydrocarbon reservoir.
In another preferred aspect of this embodiment, the composition is used in a carbon dioxide and water tertiary recovery project.
In another preferred aspect of this embodiment, the composition is used in a tertiary oil recovery process, one component of which constitutes water injection.
In another aspect, this invention is a method of increasing the mobilization or recovery rate of hydrocarbon fluids in a subterranean formation comprising injecting into the subterranean formation a composition comprising highly cross linked expandable polymeric microparticles having an unexpanded volume average particle size diameter of from about 0.05 to about 10 microns and a cross linking agent content of about 1,000 to less than about 9,000 ppm of labile cross linkers and from 0 to about 300 ppm of non-labile cross linkers, wherein the microparticles have a smaller diameter than the pores of the subterranean formation and wherein the labile cross linkers break under the conditions of temperature and pH in the subterranean formation to decrease the mobility of the composition.
In another aspect of this embodiment, the composition is added to injection water as part of a secondary or tertiary process for the recovery of hydrocarbon from the subterranean formation.
In another aspect of this embodiment, the injection water is added to a producing well. Use of the composition of this invention in a producing well increases the oil-to-water ratio of the produced fluid. By injecting a composition comprising the polymeric microparticles of this invention and allowing the particles to expand, the water producing zones can be selectively blocked off.
In another aspect of this embodiment, the injection water is added to the subterranean formation at a temperature lower than the temperature of the subterranean formation.
In another aspect of this embodiment, the subterranean formation is a sandstone or carbonate hydrocarbon reservoir.
The foregoing may be better understood by reference to the following examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.